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"This is silly and makes no sense in operating system terms. "Eject" from memory? "Discard?" Ask an OS how much memory is free and it will tell you how much is currently not allocated, period. If it's telling you 61M is in use, some program has asked for it. If some program has asked for it, the OS is not free to just discard it - it has no idea if the program is legitimately using it or just allocating stuff at random - it MUST swap it. This is why memory leaks are so serious. If a program doesn't properly deallocate the space, it remains "in use" as far as the OS is concerned." You still don't understand. Of course the operating system keeps around executables which are not in current use. At least any decent one does, and certainly all Unixes do this. If you then rerun the program before the executable gets ejected, you do not need to reload it. Tannenbaum's book is certainly the best reference, and I would suggest it as the best source (obviously your other sources if they do not deal with this elementary concept are rather lacking!) The other kind of data that is dynamic in this sense in some operating systems is disk caching information, although some OS's (e.g. OS/2) allocate a fixed amount fo phyiscval memory for this purpose. But certainly for example Netware will always be using all of physical memory, since it will use any unused memory to enlarge the disk cache. Then if you load in more stuff, disk cache blocks are ejected as needed. This idea of keeping stuff around is quite old in virtual memory systems. It was certainly used in ICL George 4 in the early 70's, and indeed I think even early unices had the concept of the sticky bit. It may be that TSS also did this, but my TSS reference manuals are not at hand.